A vehicle window antenna currently in use on motor vehicles, as seen in FIG. 1, has an antenna element 2 comprising a grid of electrically conductive frit material affixed to the inner side of a vehicle rear window glass 3 and an electrically conductive rear defogger or heating element 4 comprising a grid made of the same electrically conductive frit material and affixed to the inner side of the rear window glass below the antenna element. The window glass is retained on a vehicle body 5 to cover a window aperture defined by an aperture edge 6 of body 10, which edge is overlapped by window glass 3. The antenna and heater elements are not directly connected to each other but are spaced from each other such that the defogger element is coupled to the antenna element to be driven thereby as a parasitic antenna element. Since the heating element covers most of the viewing area of the window glass, the antenna element is confined to a small upper portion of the window glass adjacent the top and upper side portions of aperture edge 6.
Window glass 3 is held on body 10 by a mounting and sealing member, not shown, which is laid in a recessed channel formed in the vehicle body adjacent the aperture edge. The member comprises a strip of urethane material having electrically conducting properties at radio frequencies due to its dielectric constant and to electrically conductive particles included therein to provide a black color. At least a portion of the active antenna element must be sufficiently near a ground plane provided by the vehicle body to be coupled therewith; and this is accomplished in the prior art by providing a portion of the active antenna element on the inner side of the window glass near the top and upper side edges of the vehicle body aperture. Due to the facts that the mounting and sealing member (1) is very near the aperture edge in the vehicle body, (2) separates the plane of the antenna element on the window glass from that of the aperture edge, and (3) is electrically conductive at radio frequencies, it generally provides a low impedance extension of the vehicle body ground plane and comprises the closest portion of the ground plane to the antenna.
The distance between the antenna element and ground plane significantly affects the antenna impedance; and the smallest distance between the antenna element and the ground plane dominates that affect. It is thus important to control the smallest distance between the antenna and the ground plane precisely to maintain a predetermined antenna impedance. However, the mounting and sealing member is subject to a plurality of variables during the vehicle assembly process which can lead to a variation of this distance. It is generally applied by laying a bead of the urethane material in a strip around the aperture edge; and the precise path of the bead and amount of material per unit distance can vary around a single window as well as from vehicle to vehicle. The bead is easily deformable until cured; and the window glass is laid in the channel and pressed down on the mounting and sealing member, which causes the bead of material to flatten and widen. The pressure applied to the window glass may vary, which results in a variable width and height of the member. The window glass may be laid slightly out of position and moved sideways into a better position, in which case the bead may be distorted into a slanted cross-sectional shape as the top is moved by its contact with the window glass sideways with respect to the bottom. It is thus difficult in a mass assembly environment to control the final position and configuration of the mounting and sealing member with the precision required to guarantee that the closest distance between the antenna element and the vehicle body ground plane, as extended by the mounting and sealing member, will not vary from that designed to provide the predetermined antenna impedance.